This disclosure relates generally to the field of photolithography. More particularly, but not by way of limitation, it relates to a sub-wavelength photolithographic technique that overcomes the diffraction limitations of traditional photolithography.
Photolithography is a critical step in the formation of the complex electronic circuitry that drives the modern devices (such as digital memory devices, video display devices, and digital imaging devices to name a few) to which we have become accustomed. The process of creating the semiconductor chips utilized by these modern devices typically involves several iterations of the photolithographic cycle.
Photolithography takes advantage of the altered properties of a light-sensitive substance known as a photoresist upon exposure to electromagnetic (EM) radiation. These altered properties of the photoresist material allow for the selective formation of various components of microelectronic circuits with features smaller than one micrometer. In a particular example of the traditional photolithographic process depicted in FIG. 1, for example, a photoresist 102 is applied to an insulating material 104 (such as silicon dioxide), which is itself disposed on a semiconductor material 106 (such as a silicon crystal), to form a substrate 108. The photoresist 102 is typically applied by a spin coating process in which a liquid solution of the photoresist 102 is applied to the insulating material 104 while the substrate 108 is rapidly spinning. The spin coating process ensures that the photoresist 102 is applied in a uniform, flat layer.
A reticle 110 corresponding to a desired pattern is subsequently placed between an electromagnetic (EM) source 112 and the photoresist 102 such that certain portions of the photoresist 102 are exposed 114 upon activation of the EM source 112 while other portions of the photoresist 102 are unexposed 116. While the particular example illustrated in FIG. 1 depicts a simple pattern for purposes of clarity, the photolithographic process can be utilized to create complex patterns with great precision.
As described above, the properties of exposed portion 114 of the photoresist 102 are altered based on exposure to EM source 112. For example, exposed portion 114 of the photoresist 102 may be soluble in a particular solvent while unexposed portions 116 may be insoluble in the same solvent. Consequently, at step 118, the exposed portion 114 of the photoresist 102 is selectively removed while the unexposed portions 116 of the photoresist 102 remain, creating an exposed portion 120 of insulating material 104. It should be noted that the photoresist depicted in FIG. 1 is what is known as a positive photoresist, because the exposed portions of the photoresist become more soluble than the unexposed portions. Negative photoresists, in which the exposed portions of the photoresist become less soluble than the unexposed portions, are also known and utilized.
At step 122, the exposed portion 120 of insulating material 104 is removed (e.g., etched) by a known process while the portions of insulating material 104 that are protected by the remaining photoresist 102 are not removed. Subsequently, the remaining photoresist 102 is removed (e.g., dissolved in a solvent in which even the unexposed portions 116 of photoresist 102 are soluble), leaving the substrate 108 with the desired pattern. Subsequent photolithographic cycles may be utilized to further pattern the substrate 108 to form desired circuitry. While the process illustrated in FIG. 1 depicts the usage of photolithography to selectively remove material (in this case exposed portion 120 of insulating material 104), the photolithographic process is equally applicable for the selective deposition of material. Thus, photolithography allows for the precise formation of microelectronic circuitry by the selective removal and deposition of materials on a substrate.
As the desire for smaller and more efficient electronic devices increases, there is a corresponding desire to form smaller patterns using photolithography to create the electrical circuitry that will drive these devices. It is well known, however, that diffraction limits the size of the features formed by traditional photolithography methods to approximately one half of the wavelength of the EM source 112 used to pattern the features according to the Rayleigh criterion. It would seem logical, therefore, to use an EM source 112 having a shorter wavelength. However, various problems arise with respect to shorter wavelength EM radiation. For example, as is well known, the wavelength of EM radiation is inversely proportional to the frequency of the EM radiation. Moreover, the frequency of EM radiation is directly proportional to the energy of the EM radiation. Thus, as wavelength decreases, frequency and energy increase. When insulating materials 104 such as silicon dioxide are exposed to photons with an energy greater than the band gap of the material, free electrons are released, thereby adversely affecting the insulating properties of the material. Furthermore, deep ultraviolet and x-ray radiation are significantly absorbed by traditional lenses and air such that they are not practical for usage in photolithography. There is thus a need to overcome the diffraction limit such that sub-wavelength patterns can be formed via photolithography.
Several attempts to achieve this goal have been proposed. A first interferometric approach requires entangled photon number states that are experimentally difficult to generate and sustain. A second approach, based on classical light pulses, achieves sub-wavelength resolution by correlating wave vector and frequency in a narrow band multi-photon detection process. This approach is based on an N-photon absorption process and can achieve a spatial resolution of λ/(2N), where λ is the wavelength of the light. The multiphoton transition of this approach, however, is accompanied by the need for high light field intensities, which makes an experimental realization of the technique impractical. Finally, a method based on dark state physics that would achieve the same λ/(2N) resolution without the N-photon absorption process has been proposed. This scheme relies on the preparation of the system in a position dependent trapping state via phase shifted standing wave patterns and employs resonant atom-field interactions only. The method, however, requires multibeams and multilambda systems, and is therefore also not practically realizable.
There is thus a need for a photolithographic method that overcomes the diffraction limit and is realizable using current technology such that sub-wavelength patterns can be formed via photolithography.